1. Field of the Invention
The invention pertains to metal oxide semiconductor field effect transistors (MOSFETs), and more particularly to silicon metal compounds used for forming contacts to MOSFET source and drain regions and gate electrodes.
2. Related Technology
Metal oxide semiconductor field effect transistors (MOSFET) are the primary component of most semiconductor devices. FIG. 1 shows a MOSFET formed in accordance with conventional technology. The MOSFET includes a semiconductor substrate 10 in which are formed deep source and drain regions 12 and shallow source and drain extensions 14. A polysilicon gate 16 overlies a gate oxide 18 and is protected by spacers 20 formed on its sidewalls. The source and drain regions 12 are coupled to conductive contacts 22 that are inlaid in a protective layer 24 such as borophosphosilicate glass (BPSG).
The conventional MOSFET utilizes silicide regions 26 to facilitate the formation of ohmic contacts having low contact resistance with the source and drain regions 12. A silicide region 28 is also formed on the gate 16 to form a structure conventionally referred to as a polycide gate. Conventionally, a refractory metal such as titanium, tungsten, tantalum or molybdenum was used for forming gate and source/drain silicides. More recently cobalt has been adopted as the preferred metal for silicide formation. The silicide regions serve as an interface that reduces the contact resistance between the underlying silicon and other materials such as aluminum that are traditionally used as a first level of connection for sources, drains and gates.
Silicide regions such as those shown in FIG. 1 are typically made though a self-alignment process and are therefore sometimes referred to as xe2x80x9csalicidesxe2x80x9d (Self Aligned siLICIDEs). In the salicide process, the gate and gate oxide are patterned together in a self-aligned fashion using a single hardmask, a low energy dopant implantation is performed to form the shallow source and drain extensions, the spacers are then formed on sidewalls of the gate and gate oxide, high energy dopant implantation is performed to form deep source and drain regions, and then a conformal layer of a metal is formed over the entire structure. Thermal treatment is then performed to promote formation of a silicide compound comprising the metal and the silicon of the source and drain regions and the polysilicon of the gate. After performing thermal treatment for a time sufficient to produce a desired thickness of silicide, the remaining metal is removed, leaving silicide regions as shown in FIG. 1.
Demands for increased device speed and performance have driven a continuous search for ways to reduce the size and increase the operating speed of MOSFETs. At the same time, further research has been directed toward identifying new materials that may be substituted for conventional materials to provide advantages such as greater ease of miniaturization and simplified process integration.
One material that is being considered as a replacement for the conventional refractory contact metals is nickel (Ni). Nickel forms three phases of nickel silicide: metal-rich di-nickel suicide (Ni2Si), nickel monosilicide (NiSi), and silicon-rich nickel di-silicide (NiSi2). Nickel monosilicide is particularly preferred, as it has a low resistivity comparable to that of titanium, but consumes less silicon during silicide formation and therefore lends itself to silicidation of very thin active regions. Nickel monosilicide sheet resistance also has no adverse line-width dependence, which is often a problem with the traditional refractory suicides. Nickel monosilicide sheet resistance has even been found to decrease in narrow lines because of an edge effect that is specific to nickel monosilicide. The mechanical stress of nickel monosilicide on a silicon substrate is less than that of titanium. In addition, contact resistance to p-type silicon is lower than that of titanium silicide, and is also low for n-type silicon. Nickel monosilicide also exhibits good adhesion to silicon. Therefore nickel monosilicide presents a desirable substitute for conventional refractory contact metals in MOSFETs with ultra-shallow source and drain diffusions.
One drawback of nickel monosilicide is that its thermal stability is relatively poor at temperatures typically encountered during MOSFET fabrication. FIG. 2 shows a graph relating the sheet resistance of the three phases of nickel silicide compounds with their formation temperatures. As seen in FIG. 2, nickel monosilicide is best formed between 300-600 degrees C., and when formed in that temperature range it provides a sheet resistance of less than three ohms per square. In contrast, the metal-rich di-nickel silicide and the silicon-rich nickel disilicide are formed at temperatures outside of this range, and each produces a significant increase in sheet resistance. As a result, the high process temperatures typically used in semiconductor processing for purposes such as source/drain implant anneals can promote the conversion of nickel monosilicide into nickel disilicide, and thereby increase the sheet resistance of the silicide. Therefore a design that uses nickel monosilicide is restricted in the thermal budget that is available for further processing.
Consequently, the current technology does not provide a manner of exploiting the beneficial features of nickel monosilicide without significantly restricting the thermal budget available for subsequent processing.
In light of the advantages and disadvantages of nickel monosilicide as explained above, it is an object of the invention to harness the advantages of nickel as a contact metal in MOSFET source/drain and gate silicides while relaxing the thermal budget restrictions inherent in the known nickel monosilicide processes.
In accordance with an embodiment of the invention, the gate of a MOSFET is capped with a layer of polycrystalline polycrystalline silicon germanium. Silicidation with nickel is then performed to form a nickel-germanosilicide that preferably comprises the monosilicide phase of nickel silicide. The source and drain regions may also employ nickel germanosilicide. The incorporation of germanium in the silicide provides a wider temperature range within which the monosilicide may be formed, while essentially preserving the sheet resistance exhibited by nickel monosilicide. As a result, the nickel germanosilicide is capable of withstanding greater temperatures during subsequent processing than nickel monosilicide, yet provides approximately the same sheet resistance and other beneficial properties as nickel monosilicide.
In accordance with one embodiment of the invention, a MOSFET is manufactured from a semiconductor substrate having formed thereon a gate insulating layer, a polysilicon layer formed on the gate insulating layer, and a polycrystalline silicon germanium layer formed on the polysilicon layer. The polycrystalline silicon germanium layer, the polysilicon layer and the gate insulating layer are patterned to form a gate insulator and a gate overlying the gate insulator, the gate comprising a lower polysilicon portion and an upper polycrystalline silicon germanium portion. A layer of nickel is then formed over at least the upper polycrystalline silicon germanium portion of the gate, and thermal treatment is performed to form a nickel germanosilicide on the gate.
In accordance with another embodiment of the invention, a MOSFET includes source and drain regions, a channel region extending between the source and drain regions, a gate insulator overlying the channel region, and a polycide gate overlying the gate insulator. The polycide gate has a lower polysilicon portion and an upper polycrystalline silicon germanium portion, and a nickel germanosilicide formed on the upper polycrystalline silicon germanium portion. Alternatively, at least one of the polycide gate and the source and drain regions comprises nickel germanosilicide.